1. Field of the Invention
The present invention relates to multi-stage launch vehicles. In particular, the present invention relates to relatively high performance, high reliability launch vehicles for placing payloads into earth orbit and beyond, i.e., escape from the gravity of Earth.
2. Description of Prior Art and Related Information
The various approaches to launch vehicle design may be generally classified into single stage or multistage launch vehicle systems. Single stage launch vehicles employ a single thruster stage which includes all the propellant required to deliver a specified velocity to the payload. Since considerable mass is contained in the propellant tanks, engines and thrust structure, which mass becomes unnecessary once propellant therein is expended, a single stage launch vehicle is inherently of less than optimum efficiency. Multi-stage launch vehicles, where an entire stage, including propellant tanks and engines, is jettisoned after propellant expenditure, have accordingly been developed and gained predominance for earth orbit launch applications. The Titan is a vehicle having those characteristics.
The Titan was originally designed as a two-stage, liquid rocket ICBM (Intercontinental Ballistic Missile). The velocity requirements for ICBMs are substantially lower than the velocities required to place a payload into Earth Orbit. To achieve increased capabilities for this launch vehicle, updating of engine thrusts and lengthening of the stage(s) to accommodate increases in propellant have been employed. Additionally, for very large payloads, two solid rocket strap-on stages are added to the liquid rocket stages. Substantial further increases in performances of the Titan are inhibited, however, by the difficulties associated with achieving further increases in engine liquid rocket thrusts and increased stage lengths. In particular, the stage length problem is severe since the ratio of the overall length to diameter of a launch vehicle is critical to its stiffness, which in turn, is critical to the dynamic loads it can withstand due to high altitude winds that it encounters as it traverses the Earth's atmosphere. Also, the wind loads increase as the length of the vehicle increases. Similar problems are also presented with providing increased payload to Earth Orbit capabilities for the other expendable U.S. launch vehicles, the Atlas and Delta launch vehicles, since these were also originally designed as ICBMs or IRBMs (Intermediate Range Ballistic Missiles).
These difficulties in providing further upgrades in capability may be appreciated by consideration of one specific upgraded Titan launch vehicle, the Titan IV. This upgraded Titan is one of three current unmanned expendable U.S. Space Launch Vehicles. The others are the Atlas II and Delta II. A Titan IV configuration is shown in FIG. 1.
The Titan IV can be flown as a three or four stage launch vehicle for space missions in Low Earth Orbit (LEO) and High Energy Orbits (HEO), i.e., Geosynchronous Orbits (GEO). The first stage consists of two uprated solid rocket motors (SRMU) boosters 1 and 2 "strapped-on" to the second and third liquid propellant stages, 3 and 4. The three stages operate in series, i.e. the thrust of each stage is initiated after previous thrusting stages have expended their propellants and been staged. The total thrusts delivered by the SRMU boosters 1 and 2 at liftoff is approximately 2.8 million lbf. For GEO missions, the fourth stage 5 is a modified Centaur stage propelled by two LO.sub.2 /LH.sub.2 cryogenic engines. The four stage vehicle has a performance of approximately 12,000 lbs to GEO. The Titan IV liquid core stages 3 and 4 have been uprated in performance by increasing engine thrusts and stage lengths to accommodate more propellant while maintaining their original ten foot diameters. The length to diameter ratio of a launch vehicle is critical to its stiffness, which in turn is critical to the dynamic loads it can withstand due to high altitude winds it encounters as it traverses the Earth's atmosphere. The wind loads must be limited both to protect the structural integrity of the vehicle and to maintain its control authority by means of the engines thrusting at varying gimbal angles to maintain the proper vehicle attitude. Small increases in performance have been achieved by employing load alleviating flight trajectories which reduce the wind loads. Although stiffening the Titan liquid core (stages) is a possibility, the design changes required to achieve a substantial improvement would be extensive. Thus, it is clear that large performance increases will not be achieved without substantial increases in the stiffness of the liquid stages 3 and 4 and increases in engine thrust either by new engine developments or increasing the numbers of existing engines in the vehicle.
Titan IV performance could be improved by igniting the liquid second stage 3 at liftoff, thus providing a first and second stage parallel burn from lift off. However, this would require an increase in the second stage 3 length to accommodate the propellant burned during first burn. This would exacerbate the core stages length to diameter ratio problem. Also, in addition to performance limitations, all of the engines in Titan IV (and Atlas II and Delta II) must function properly to achieve a successful launch. As a result, most Titan IV launch vehicle failures, as well as Atlas II and Delta II failures, are due to one of the engine's failing to thrust. This problem cannot be dealt with by adopting an engine-out strategy because none of these vehicles have a sufficient number of engines to meet their mission performance requirements when an engine fails (engine-out capability).
Accordingly, there presently exists a need to improve the performance, reliability and cost effectiveness of one or more of the current U.S. expendable launch vehicles.